Method of assembling electronic components of an electronic system, and system thus obtained

ABSTRACT

An electronic system comprising: an electronic system support substrate for the attachment of components of the electronic system, the electronic system support substrate including electric signal propagation paths for the propagation of electric signals between the system components; at least a first and a second electronic components wherein at least the first electronic component is part of a module in mechanical and electrical connection with the electronic system support substrate, the module comprising a module substrate to which the first electronic component is at least mechanically connected, and an electric coupling between the first and the second electronic components, for the electric coupling allowing the first and the second electronic components exchange of electric signals. The electric coupling comprises a direct electric connection particularly formed by a flexible electrical interconnection member, between the first and the second electronic components, the electric connection being independent of the electronic system support substrate.

TECHNICAL FIELD

The present invention relates to the field of electronic circuits andsystems, particularly albeit not limitatively to systems including bothelectronic and optical components (hereinafter shortly referred to asopto-electronic systems). Specifically, the present invention relates tothe aspects of packaging, assembling and functionally interconnectingthe different systems' components.

BACKGROUND ART

Components of electronic or opto-electronic systems, such as for exampleIntegrated Circuits (ICs) like data and/or signal processors(microprocessors, CPUs, DSPs, ASICs), photodetectors, semiconductorlasers, are usually packaged in respective packages, and then assembledto boards (Printed Circuit Boards—PCBs) that perform a double functionof providing a mechanical support as well as a functional (electrical)interconnection between the different systems' components.

One packaging solution known in the art, featuring a high packingefficiency, is the so-called Multi-Chip-Module (MCM) packaging: in anMCM package, two or more IC chips (dies) are directly attached,typically soldered (for example, using so-called C4—Controlled CollapseChip Connections—technique) to a common substrate, the MCM substrate,e.g. of organic material; the MCM package, rather than the individual ICchips, is then assembled to the PCB.

Electronic components' packages has the primary function of protectingthe packaged components mechanically and from attacks by agents in theexternal environment.

However, in applications involving high operating speeds, which arebecoming more and more common, a further requirement of the packages isthat they essentially maintain the performance levels of the electroniccomponents they carry within, or that they affect the packagedcomponents' performances as less as possible. An example of high-speedapplication where the characteristics of components' packages arecritical are opto-electronic systems, wherein high switching speedelectrical signals often have to be converted into optical signals, andvice versa.

In particular, when the signals' switching speed exceeds the Gigahertz,a proper analysis of the electronic system needs to take into accountthe wave nature of the electromagnetic field: the transmission of anelectric signal (e.g., a voltage) needs to be considered from theviewpoint of an electromagnetic wave that propagates through thecircuit, being supported by an electric current in a circuit'sconductive trace.

The solutions adopted for packaging the electronic components of ahigh-speed system affect the propagation of the electromagnetic wave.

An important role is played by the properties of the packages'materials: it is for example known that the materials' dielectricconstants and dielectric losses affect the electromagnetic wavepropagation. Another aspect that impacts the performance of the packagedcomponents is the package structure (such as its spatial configuration).

As a result, a package, if not carefully designed and selected, may havesuch an impact on the packaged component's performance (e.g., thepackage may affect the propagation of electromagnetic wavescorresponding to the signals generated or received by the packagedcomponent to such an extent) that the packaged component becomes almostinoperable, at the expected operating speed.

For example, considering the case of an opto-electronic system,light-emitting devices (e.g., laser diodes) used to convert electricalsignals into optical signals need to receive electrical signals alreadymodulated at high speed, generated for example by a microprocessor: abad electrical signal transmission from the signal generator to theelectro-optical converter translates into a bad optically convertedsignal. Similar considerations apply to the reverse signal conversion,from optical into electrical: the high-speed electrical signalsgenerated by, e.g., a photodetector, like a photodiode, must not beworsened too much in the propagation from the photodetector to the IC(s)that have to process the converted electrical signals.

Packages for components of electronic systems thus need to be designedin such a way that they do not affect, as far as possible, thepropagation of electromagnetic waves associated with the electricsignals generated/received by the packaged ICs.

To this purpose, a known countermeasure calls for designing andrealizing circuit structures having a carefully controlled impedancevalue across a generic signal transmission line.

Controlling the transmission line impedance value is however notsufficient, due to the unavoidable presence of parasitic elementsexhibiting a capacitive, resistive or inductive behavior, whichparasitic elements are intrinsically embedded in the package, or in thePCBs, due to the association of materials and conductive structuresneeded to establish paths for electrical currents.

A careful selection of materials with physical properties favorable tothe electromagnetic wave propagation, such as for example PTFE(PolyTetraFluoroEthylene), is not sufficient to compensate and overcomeall the other effects, inherent to the package structure, e.g. thespatial configuration of MCM structures.

In particular, the propagation of electromagnetic waves is severelyaffected by any kind of physical discontinuity along the wavepropagation path; by physical discontinuity there is intended any moreor less abrupt change or transition in properties such as structure,material properties, design features.

For example, let the case be considered of an electronic system whereinelectric signals for driving an electro-optical component, like a laserdiode, particularly a VCSEL (Vertical Cavity Surface Emitting Laser),are generated by an IC, e.g. a CPU, which is packaged in an MCM package,and assembled to a PCB to which the VCSEL is also mounted. Severaldiscontinuities can be observed in the signal path from the signalsgenerator IC to the laser diode, namely the transitions from the ICsignal line to the corresponding IC pad, from the IC pad to the (e.g.,C4) solder bump, from the solder bump to the corresponding (e.g., C4)contact pad on the MCM's substrate, then to the conductor signal line onthe MCM's substrate, from the MCM's conductor signal line to the MCM'sbondage pad (e.g., a Ball Grid Array—BGA—pad) used for bonding the MCMsubstrate to the PCB (this transition may in particular be made up ofseveral different transitions, corresponding for example to one or morelaser vias and PTHs—Pin Through Holes), from the MCM's bondage pad tothe (e.g., BGA) solder bump and to the corresponding (e.g., BGA) contactarea on the PCB, then to the signal line trace on the PCB up to thelaser diode. Some of the above transitions have an inherent impedancemismatch.

Experiments have demonstrated that the transition corresponding to theBGA-type bondage of the MCM substrate to the PCB has the biggest impactat high operating frequencies, having an essentially capacitive effect,and thus acting as a low-pass filter that significantly reduces thetransmission line bandwidth. A lower importance, but not negligible roleis played by plated through-holes in the MCM's substrate or in the PCB,C4 pads for bonding the IC chips to the MCM's substrate, laser vias andcoupling effects between the signal lines and voltage supply planes.

SUMMARY OF THE INVENTION

The Applicant has faced the problem of how to reduce to impact ofsignals' propagation in electronic or opto-electronic systems on thesignals' properties.

The Applicant has devised a new solution for assembling and,particularly, functionally connecting to each other components of anelectronic system, that allows to substantially reduce the number ofdiscontinuities along a signal propagation path; thus, thanks to thesolution devised by the Applicant, the propagation of the signalsslightly affects the signals' characteristics.

According to an aspect of the present invention, an electronic system isprovided, as set forth in appended claim 1, comprising:

an electronic system support substrate for the attachment of componentsof the electronic system, said electronic system support substrateincluding electric signal propagation paths for the propagation ofelectric signals between the system components;

at least a first and a second electronic components, wherein at leastthe first electronic component is part of a module in mechanical andelectrical connection with the electronic system support substrate, saidmodule comprising a module substrate to which the first electroniccomponent is at least mechanically connected, and

an electric coupling between the first and the second electroniccomponents, for the electric coupling allowing the first and the secondelectronic components exchange of electric signals,

said electric coupling comprises a direct electric connection betweenthe first and the second electronic components, said electric connectionbeing independent of the electronic system support substrate.

In particular, said electric connection includes a flexible electricalinterconnection member having a first end electrically connected to thefirst electronic component, and a second end electrically connected tothe second electronic component.

According to a second aspect of the present invention, a method as setforth in appended claim 15 is provided for, for assembling an electronicsystem, comprising:

providing an electronic system support substrate for the attachment ofcomponents of the electronic system and including electrical signalpropagation paths for the propagation of electrical signals between thesystem components at least;

providing a first and a second electronic component, wherein at leastthe first electronic component is attached to a module comprising amodule substrate to which the first electronic component is at leastmechanically connected,

assembling the first and second electronic components to the electronicsystem support substrate, said assembling including establishing anelectric coupling between the first and the second electronic componentfor the exchange of electric signals,

said establishing an electric coupling comprises providing a directelectric connection between the first and the second electroniccomponents, said electric connection being independent of the electronicsystem support substrate.

In particular, said providing said electric connection includes aproviding flexible electrical interconnection member having a first endand a second end, and electrically connecting the first end to the firstelectronic component, and the second end to the second electroniccomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be madeapparent by the following detailed description of some embodimentsthereof, provided merely by way of non-limitative examples, descriptionthat will be conducted making reference to the attached drawings,wherein:

FIG. 1 schematically depicts a small portion of a PCB of an electronicsystem with, mounted thereto, an MCM package carrying an IC electricallyconnected to an electro-optical conversion component by means of aflexible electrical interconnection member, according to an embodimentof the present invention;

FIG. 2 shows a detail of the flexible electrical interconnection memberof FIG. 1, in an alternative embodiment of the present invention;

FIGS. 3 and 4 schematically show, in top-plan and cross-sectional views,respectively, a terminal portion of a flexible electricalinterconnection member, according to an embodiment of the presentinvention;

FIG. 5 schematically shows, in top-plan view, two ends of two spans of aflexible electrical interconnection member intended to be joined to eachother, according to an embodiment of the present invention;

FIGS. 6 and 7 schematically show, in top-plan and cross-sectional views,respectively, two ends of two spans of a flexible electricalinterconnection member intended to be joined to each other, according toan alternative embodiment of the present invention;

FIG. 8 is a view similar to that of FIG. 2, but showing a furtherembodiment of the present invention; and

FIG. 9 is a simplified, schematic top-plan view of the embodiment ofFIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

With reference to the drawings, in FIG. 1 there is shown, veryschematically, a portion of a PCB 100 of an electronic system; forexample, the electronic system may include several PCBs like the PCB100, adapted to be connected to a backplane (not shown in the drawings)of a rack. The electronic system may for example be part of an opticalcommunications system.

As visible in the drawing, an MCM package 105 is mounted to the PCB 100;schematically, the MCM package comprises an MCM substrate 110, made forexample of a ceramic or organic (e.g., plastic) material. Two or more ICchips are mounted to the MCM substrate 110, like the two IC chips 115 aand 115 b shown in the drawing. The IC integrated in the chips mountedto the MCM substrate 110 can be of whatsoever type, their specificnature being not critical nor limitative to the present invention; inparticular, the ICs can be standard or full-custom or ASIC chips, likefor example a microprocessor IC (a CPU), a DSP IC, a memory IC, or thelike.

The IC chips 115 a and 115 b are attached to the MCM substrate 110 bymeans of suitable mechanical and electrical connections. For example, ICcontact pads (not visible in FIG. 1) are connected by means ofgenerically ball-shaped solder bumps 125, made of a suitable solderalloy, to corresponding contact pads 120 on the upper surface of the MCMsubstrate 110, for example by means of C4 (Chip Controlled CollapseConnection) technique. Other bonding techniques are possible, forexample thermosonic, ultrasonic or thermocompression wire-bonding (aswill be discussed later in the present description) or mixed techniques,as well asbonding techniques that in place of ball-shaped solder bumpsuse vertically-extending elements like small pillars made of copper ordifferent solder alloys, protruding, or being attached to the IC pads,thus establishing the interconnection mechanism.

In the MCM substrate 110, conductor lines 130 are formed of a conductormaterial, like for example: plated copper (typically used in organicrigid and/or flexible substrates, as well as in ceramic packages),molibdenum (typically used in thin-film ceramic packages), aluminum orcopper (typically used in thin-film technologies) using a carrier ofsilicon, quartz, borosilicate glass, lead borosilicate, boron nitride,diamond; thick-film technologies typically uses silver, gold, copper,gold-platinum, silver-palladium, and palladium-gold. In particular, theconductor lines 130 may be formed only on the top and bottom surfaces ofthe MCM substrate 110, or the MCM substrate 110 can have a multi-layeredstructure, with one or more intermediate planes of conductor lines 130;vias 135 in the MCM substrate 110 allow electrically connectingconductor lines 130 located on the two opposite MCM surfaces and/or inthe one or more intermediate planes. The conductor lines 130 may inparticular be provided for operatively connecting to each other thedifferent IC chips mounted to the MCM substrate 110, as well as forconnecting (at least some of the contact pads of) the IC chips mountedto the MCM substrate to contact pads 140 provided on the MCM package,intended for connecting the MCM package to the PCB 100; in particular,according to an embodiment of the present invention, supply voltages andlow switching frequency signals are transferred in this way from the PCBto the MCM, or vice versa.

A cap or an heat-sink 145 may be provided to cover the MCM substrate110, protecting the IC chips of the MCM and also providing some level ofsealing.

The MCM package 105 is attached to the PCB 100 by means of solderingtechniques; in particular, and merely by way of example, the MCM pads140 may be arranged according to a BGA arrangement, and they may bebonded to corresponding PCB contact pads 150 by means of BGA solderbumps 155, according to specific custom solution or to internationalindustry standards like the JEDEC™ MS034 of the JEDEC Solid StateTechnology Association (formerly, Joint Electron Device EngineeringCouncil). Alternative soldering techniques involve the usage of verticalstructures such as columns or pillars made of soldering alloys or metalslike copper, soldered or brazed to the module.

In the PCB 100, conductor traces 160 are formed of a conductor material;in particular, the conductor traces 160 may be formed only on the topand bottom surfaces of the PCB 100, or, more typically, the PCB 100 canhave a multi-layered structure, with one or more intermediate planes ofconductor traces 160; plated through holes 165 in the PCB 100 allowcontacting non-coplanar conductor traces 160, that lie on differentplanes. The conductor traces 160 may in particular be provided foroperatively connecting two or more MCM packages to each other, or to ICchips or other components mounted directly on the PCB.

Also schematically shown in the drawing is an electro-optical converter170, adapted to convert an electrical signal, e.g. a voltage signal,into a corresponding optical signal, to be propagated for examplethrough a waveguide, like an optical fiber 185. In particular, andmerely by way of example, the electro-optical converter 170 comprises aVCSEL 175 and an associated Laser Diode Driver (LDD), optically coupledto a standard optical connector 180, for example for the connection ofan optical fiber 185. The electro-optical converter 170 is arrangedclose to an edge 187 of the PCB 100 (a typical solution).

For the purposes of describing an exemplary embodiment of the presentinvention, in the following it will be assumed that one of the chips inthe MCM package 105, for example the chip 115 a, has integrated thereinan IC intended to be operatively associated with, particularly drivingthe electro-optical converter 170; for example, the IC may be aprocessor, e.g. a CPU that generates electrical signals (characterizedby a high switching speed, of 1 Ghz or more), e.g. voltage signals,which are to be fed to the electro-optical converter 170 for beingconverted into corresponding optical signals, to be then propagatedthrough the optical fiber 185.

The signals to be fed to the electro-optical converter 170 are madeavailable at respective contact pads of the IC chip 115 a, for example(referring to the enlarged detail of FIG. 2) at the pads 200 a and 200 bof the IC chip 115 a.

In order to reduce the signal path from the pads 200 a and 200 b to theelectro-optical converter 170, the latter is preferably arranged asclose as possible to the IC chip 115 a.

In prior-art solutions, as discussed in the introductory part of thepresent description, the signal path from the IC chips pads 200 a and200 b to the corresponding terminals of the electro-optical converter170 would include several transitions, with associated physicaldiscontinuities that would have a strong, negative impact on thepropagation of the electromagnetic waves corresponding to thepropagation of the high-speed signals generated by the IC integrated inthe chip 115 a. In particular, referring to the example considered anddepicted in the drawings, a solution according to the prior art wouldinvolve transitions from the IC chip signal lines to the IC pads, to theC4 solder bumps 125, to the C4 pads 120 on the MCM's substrate 110, tothe conductor lines 130 on the MCM's substrate, to the laser vias 135(multiple such transitions may occur, in multi-layered MCM substrates),to the BGA bonding pads 140 of the MCM, to the BGA bumps 155, to the BGApads 150 on the PCB 100, to the signal trace 160 on the PCB and,finally, to the terminals of the VCSEL 170.

According to an embodiment of the present invention, instead ofconventionally propagating the high-speed electrical signals, from thesignals' generator component, e.g. the IC pads 200 a and 200 b of the ICchip 115 a, to the intended destination, e.g. the electro-opticalconverter 170, going from the signal generator's chip to the MCM, thento the PCB, and thus incurring all the transitions and physicaldiscontinuities schematically listed above, the electric terminals ofthe signals' generator component are directly connected to correspondingelectric terminals at the intended destination, e.g. electric terminalsof the VCSEL 170, and for such a direct connection a small, flexibleelectrical interconnection member 190 (like a flexible flat cable) isexploited, bypassing the above-mentioned transitions and allowing topropagate the high-speed signals along a clean and short electricalpath, possibly the shortest possible electrical path.

In other words, by using the flexible electrical interconnection member190 to electrically connect, directly, the terminals of the signals'generator component 115 a to the corresponding terminals ofelectro-optical converter 170, it is possible to avoid the necessity ofdesigning a signal path passing through all the different hierarchies ofpackaging (chips, MCM, PCB).

In particular, according to an embodiment of the present invention, theflexible electrical interconnection member 190 is connected to the ICpads 200 a and 200 b through the C4 solder bumps 125 a and 125 b (FIG.2); in other words, the IC pads 200 a and 200 b, instead of (or inaddition to) being bonded, through C4 solder bumps, to C4 pads on theMCM substrate 110, are bonded to (respective conductor strips of) theflexible electrical interconnection member 190.

In alternative embodiments of the present invention, the flexibleelectrical interconnection member 190 may be bonded to contact pads onthe MCM substrate, which pads are in turn connected, through conductorlines 130, e.g. on the upper MCM substrate surface, to, e.g., C4 contactpads 120 on the MCM substrate 110 to which the relevant pads of the ICare bonded, e.g. by conventional C4 bonding technique.

In the cited alternatives, the electrical signal path in the MCMsubstrate is minimal (particularly, under the viewpoint of thetransitions and physical discontinuities that are detrimental to thepropagation of the associated electromagnetic wave) or possibly evenreduced to zero, in the case the flexible electrical interconnectionmember is bonded directly to the (C4 solder bumps attached to the) ICpads; in any case the PCB 100 may be not involved in the signalpropagation, thus, thanks to the solution according to the describedembodiment of the present invention, all the physical discontinuitiesassociated with the transitions from the MCM to the PCB are eliminated.Additionally, the flexible electrical interconnection member 190 can beproperly selected in such a way that its properties (particularly theproperties of the materials it is made of), from the viewpoint of theimpact on the propagation of the electromagnetic wave associated withthe electrical signal, are significantly better than those of the MCMand PCB.

In FIGS. 3 and 4 a portion 300 of the flexible electricalinterconnection member 190 is schematically depicted, in an embodimentof the present invention. In particular, the flexible electricalinterconnection member portion 300 depicted in the drawings is theterminal portion thereof intended to be soldered to the (C4 bumps ofthe) IC pads of the IC chip 115 a (in FIGS. 3 and 4 the flexibleelectrical interconnection member 190 is shown detached from the ICpads, for better clarity). A similar arrangement may be provided at theopposite end of the flexible electrical interconnection member (the endconnected to the VCSEL 170). In particular, FIG. 3 is a top-plan view,whereas FIG. 4 is a sectional view of the flexible electricalinterconnection member, taken along line IV-IV. The flexible electricalinterconnection member 190 has, embedded therein, a number of conductorstrips, like the four conductor strips 305-1, 305-2, 305-3 and 305-4 inthe shown example. Referring to FIG. 4, the conductor strips 305-1,305-2, 305-3 and 305-4 are for example made of a thin film of copper,and are immersed in an insulating material 310, like for exampleBenzo-Cyclo-Butene (BCB) or Liquid Crystal Polimers (LCP), polyesters(PET) like polyethyleneterephtalate, polyimides (PI) like the onesattained by the polycondensation polymer from 4,4′bis(aminophenyl)oxideand pyromellitic dianhydride (also called ODA-PMDA-based polyimide)commercially known under the name of Kapton™ by Du-pont, orpolyperfluorocarbons like polytetrafluoroethylene (PTFE) also known asTeflon™, aramids; all the aforementioned materials can be used toproduce composite materials with different fillers like ceramic, glass,or in association to adhesives, which are used to bond flexiblesubstrates, including acrylics, epoxies, butyral-phenolics, polyesters,silicones, urethanes, fluorocarbons and blends of these materials.Similarly, recent developed materials have gained attention due to theirgood electrical perfromance like the Annylated-Poly-Phenylene-Ether(APPE).

Also, the flexible electrical interconnection member may be furthercovered with an additional dielectric/insulating layer, of materialsthat are flexible in nature and may present photoimageablecharacteristics. Basically, these materials are flexible solder masksand their utilization is aimed, wherever their chemical and physicalcharacteristics allow, to reduce the overall application cost. They canbe used to cover the flexible electrical interconnection memberprotecting its features in place of the more expensive base materialsused in the core construction of the flexible circuit itself.

The conductor strips 305-1, 305-2, 305-3 and 305-4 have exposedterminations 315, not covered by the insulating material 310; theterminations 315 are in particular enlarged for facilitating the bondingto the C4 solder bumps of the IC chip pads. For example, the twoconductor strips 305-2, 305-3 may be connected, at their enlargedterminations, to pads of the IC chip 115 a at which a differentialsignal is made available for driving the VCSEL; the two conductor strips305-1, 305-4 are for example connected to a reference voltage (e.g., theground), for shielding purposes.

A possible assembling operations sequence may be the following.

The electro-optical converter 170 is firstly assembled as a stand-alonecomponent, adopting conventional assembling techniques, and thecomponent is tested.

In parallel to the assembling and testing of the electro-opticalconverter 170, the MCM 105 is assembled (e.g., the chips are attached tothe MCM substrate 110), adopting conventional assembling techniques(e.g., C4 soldering of the chips to the MCM substrate). In the MCMassembly phase, one end of the flexible electrical interconnectionmember 190 is bonded either directly to the IC chip pads, through the C4solder bumps, or to contact pads on the MCM substrate which are in turnconnected to the proper IC pads. The assembled MCM is then tested.

Then, the assembled MCM, with a protruding pigtail of the flexibleelectrical interconnection member 190, is assembled to the PCB 100, andthe PCB is tested (particularly, an in-circuit test is carried out).

The electro-optical converter 170 is then mounted to the PCB 100, andthe flexible electrical interconnection member 190 is bonded to theelectro-optical converter 170.

The complete PCB is then tested, particularly it is functionallyverified.

An alternative sequence is the following.

The electro-optical converter 170 is firstly assembled as a stand-alonemodule, adopting conventional assembling techniques, and the componentis tested. In the assembly phase, one end of the flexible electricalinterconnection member 190 is bonded to the terminals of theelectro-optical converter 170.

In parallel to the assembling and testing of the electro-opticalconverter 170, the MCM 105 is assembled (e.g., the chips are attached tothe MCM substrate 110), adopting conventional assembling techniques(e.g., C4 soldering of the chips to the MCM substrate). The assembledMCM is then tested.

Then, the assembled MCM is assembled to the PCB 100, and the PCB istested (particularly, an in-circuit test is carried out).

The electro-optical converter 170, with the flexible electricalinterconnection member 190 attached thereto, is then mounted to the PCB100, and the flexible electrical interconnection member 190 is bonded tothe contact pads on the MCM substrate that are connected to the properIC pads.

The complete PCB is then tested, particularly it is functionallyverified.

In the invention embodiment up to now considered, a continuous flexibleelectrical interconnection member has been assumed to be used forconnecting the IC to the electro-optical converter. This solution,albeit advantageous, may pose some problems in particular situations.For example, the assembling operations sequence of a part of the finalelectronic system may involve treatments in conditions that are notsuitable for other parts of the system; for example, the electro-opticalconverter 170 may be not capable of sustaining the relatively hightemperatures required for assembling the MCMs to the PCB. In such cases,it is not possible to complete the connection of the electroniccomponents by the flexible electrical interconnection member until thecritical operations are performed. This is the reason why, in the twoassembling sequences described above, the MCM and the optical componentwere not connected to each other through the flexible electricalinterconnection member before bonding the MCM to the PCB.

In particular, in the first assembling sequence described above, one endof the flexible electrical interconnection member was bonded to the ICpads at an earlier step, during the bonding of the chips to the MCMsubstrate; however, in doing so, a flexible electrical interconnectionmember pigtail remains attached to the MCM, until at the end of theassembling sequence the other end of the flexible electricalinterconnection member is bonded to the electro-optical converter;handling pigtails may be burdensome. This problem is not present in thesecond assembling sequence described above, but in that case theelectrical signal path that is finally obtained is not as good as in theformer case, because there is a signal path portion on the MCM substrate(from the IC pads to the pads on the MCM substrate).

An alternative embodiment of the present invention that allowsovercoming the drawbacks (encountered in those cases where it is notpossible to preliminary interconnect the components to each other by theflexible electrical interconnection member and then attaching thecomponents to the PCB) of having flexible electrical interconnectionmember pigtails to be handled, while fully exploiting the excellentproperties of the flexible electrical interconnection member is depictedschematically in FIG. 2 and, in enlarged scale, in FIG. 5. The flexibleconnector 190 that directly connects the IC chip 125 a to theelectro-optical converter 170, is formed by two flexible electricalinterconnection member spans 190 a and 190 b, one, namely the first span190 a, being attached to the IC chip 115 a, and the other, second span190 b, being attached to the electro-optical converter 170. The twoflexible electrical interconnection member spans 190 a and 190 b aremechanically and electrically joined together, at respective free ends500 a, 500 b thereof. In particular, as schematically shown in FIG. 5,each conductor strip 305 a, 305 b in the first, respectively secondelectrical interconnection member span 190 a, 190 b terminates, incorrespondence of the flexible electrical interconnection member span'send 500 a, 500 b, with an enlarged pad area 505 a, 505 b, having anexposed surface free of insulating material 310, and facilitating theoperation of joining the two spans. Preferably, if the pitch of theconductor strips 305 a, 305 b is small, particularly of the order of thesize of the enlarged pad areas, the enlarged pad area 505 a, 505 b ofadjacent conductor strips are formed longitudinally displaced.

For mechanically and electrically attaching the flexible electricalinterconnection member span 190 b to the span 190 a, small solder bumps510, particularly micrometric, generically spherical or semi-sphericalbumps like micro BGA solder bumps are provided, e.g. on the enlarged padareas 505 a. Then, during the assembling, the two flexible electricalinterconnection member spans are put close to each other, particularlythe free end portion of the second span 190 b is positioned so as tosurmount the free end portion of the first span 190 a, having care tocarefully align the enlarged pad areas 505 a and 505 b. The solder bumps510 are then caused to reflow, by applying a suitable heat and pressure,for example using a thermode or a laser soldering technique, thus firmlyjoining electrically and mechanically the two flexible electricalinterconnection member spans 190 a and 190 b.

A possible assembling operations sequence may in this case be thefollowing.

The electro-optical converter 170 is firstly assembled as a stand-alonemodule, adopting conventional assembling techniques, and the componentis tested. In the assembly phase, one end of the second span 190 b ofthe flexible electrical interconnection member 190 is bonded to theterminals of the electro-optical converter 170, leaving the opposite end(the end denoted 500 b in FIG. 2) free.

In parallel to the assembling and testing of the electro-opticalconverter 170, the MCM 105 is assembled, and in particular the chips 115a and 115 b are attached to the MCM substrate 110, adopting conventionalassembling techniques (e.g., C4 soldering of the chips to the MCMsubstrate). In this assembly phase, one end of the first span 190 a ofthe flexible electrical interconnection member 190 is bonded directly tothe IC chip pads, through the C4 solder bumps 125 a, 125 b. Expediently,the first span 190 a of the flexible electrical interconnection member190 extends just to the edge of the MCM substrate 110, withoutsubstantially protruding therefrom, so that the MCM 105, once assembled,does not exhibit pigtails. The assembled MCM 105 is then tested. Theflexible electrical interconnection member span 190 a may also beanchored to the MCM substrate 110.

Then, the assembled MCM is assembled to the PCB 100, and the PCB istested (particularly, an in-circuit test is carried out).

The electro-optical converter 170, with the attached flexible electricalinterconnection member span 190 b is then mounted to the PCB 100. Theend 500 b of the flexible electrical interconnection member span 190 battached to the electro-optical converter 170 is then bonded to the end500 a of the flexible electrical interconnection member span 190 a onthe MCM 105. The small, micro BGA solder bumps 510 are provided on theenlarged pad areas 505 a of the flexible connector span 190 a, forexample by preplating the parts with the selected solder or conductiveglue media; these media can be applied by different methods, like forexample electrolytic or electroless metal plating, paste screening,molten solder injection or dispense, sputtering and so on. Then, thefree end portion of the second span 190 b is positioned so as tosurmount the free end portion of the first span 190 a, having care toalign the enlarged pad areas 505 a and 505 b. The solder bumps 510 arethen caused to reflow, by applying a suitable combination of heat andpressure, for example using a thermode or a laser soldering technique,thus firmly joining electrically and mechanically the two flexibleconnector spans.

The complete PCB is then tested, particularly it is functionallyverified.

The Applicant has observed that despite the presence of the jointjoining the two flexible electrical interconnection member spans 190 aand 190 b, the signal propagation properties of this solution arenevertheless better than those achievable in the case even a part of thesignal propagation path is on the MCM substrate, not to say on the PCB.The physical discontinuity created by the joint between the two flexibleelectrical interconnection member spans, being the joint miniaturized,is of minimal impact on the signal propagation, particularly theimpedance mismatch is low.

Under the point of view of minimizing the impact of the physicaldiscontinuity created by the flexible electrical interconnection memberspans joint, an improvement to the use of the micro spherical orsemi-spherical solder bumps is described in the following, makingreference to FIGS. 6 and 7.

Each of the two flexible electrical interconnection member spans 190 aand 190 b has, in correspondence of the free end 505 a and 505 bthereof, a portion in which the conductor strips 305 a and 305 b are notcovered by the dielectric material 310. No enlarged pad areas areprovided, differently from the embodiment of FIGS. 3 to 5. Inparticular, albeit this not a to be intended as a limitation to thepresent invention, the flexible electrical interconnection member spans190 a and 190 b include a conductive material layer 700 a and 700 b,adapted for example to be connected to a reference voltage (ground), andacting as a reference voltage (ground) plane.

During the manufacturing of the flexible electrical interconnectionmember spans, a small, controlled amount of solder alloy (whosecomposition can be of type known in the art) is provided on the exposedsurface portion of the conductor strips 305 a or 305 b of either one ofthe two flexible electrical interconnection member spans 190 a, 190 b,for example the span 190a. In particular, the amount of solder may becontrolled by exploiting a self-limiting solder transfer process, selflimited by the solder's surface tension, or by wetting.

In the electronic system assembling phase, the two ends 505 a and 505 bof the flexible electrical interconnection member spans 190 a and 190 bare aligned and stacked one onto the other, in such a way that theexposed portions of the conductor strips 305 a and 305 b are properlyaligned and superimposed one to the other. A suitable combination ofpressure and temperature is then locally applied to the stacked ends ofthe flexible electrical interconnection member spans, for example usinga flat thermode, so as to cause the solder alloy present on the exposedsurface of the conductors of one of the two spans to reflow and wet theexposed surface of the conductor strips of the other span. The heatingis merely local, and does not compromise any of the electroniccomponents already assembled to the PCB; thus, this operation can safelybe performed at the end of the assembling process, or even in the field,after the components have been assembled/replaced to the PCB. Ifdesired, the flexible electrical interconnection member span 190 b maybe anchored to the MCM substrate 110, e.g. along the edge thereof;however, it is observed that the flexible electrical interconnectionmember 190 and the joint between its two spans 190 a and 190 b is notexpected to be exposed to significant mechanical stresses that can leadto fatigue failure of the joint. Furthermore, the degree of freemovement allowed by the fact that the connection made by the electricalinterconnection member is flexible allows to compensate a much largerassembly/fabrication range of cumulative tolerance. It is also to beunderlined that the optical device 170 connected with the flexibleelectrical interconnection member 190 can be spatially placed in athree-dimensional position in respect of referenced position of theelectronics module and PCB card.

In this way, a very thin soldering 710 is achieved (through essentiallyflat solder pads), of height substantially equal to (twice) thethickness of the dielectric 310. This allows reducing the impact of thediscontinuity inherent to the joint of the two flexible conductor spans,and in particular to reduce the impedance mismatch.

FIGS. 8 and 9 show a still further alternative embodiment of the presentinvention, wherein the flexible electrical interconnection member 190,particularly the first span 190 a thereof, as in one of the embodimentspreviously discussed, is connected to the relevant pads of the IC chipthrough conventional wire-bond techniques. In particular, as schematizedin the drawings, the IC chip 115 a, instead of being attached to the MCMsubstrate 110 by means of the C4 technique (with the IC chip turnedupside-down, so that the IC bonding pads directly faces thecorresponding pads on the MCM substrate) is attached to the MCMsubstrate 110 by the bottom surface (i.e., the surface opposite to thaton which the IC pads 800 lay), for example by gluing or other attachmentmethodology; in the drawings, reference numeral 805 denotes a glue thatmaintains the IC chip 115 a attached to the MCM substrate 110. The ICchip pads 800 are wire-bonded, by means of tiny bonding wires 810,either to bonding pads/conductive strips 900 provided on the surface ofthe MCM substrate 110, or to the conductor strips 305 a, at exposedportions thereof provided at the end of the first span of the flexibleelectrical interconnection member span 190 a. Preferably, the terminalportion of the flexible electrical interconnection member span 190 a isplaced and anchored into the required position by exploiting the sameglue 805 that holds the IC chip 115 a in place; in this way, theflexible electrical interconnection member span 190 a becomes anintegral part of a device bonding pattern present on the surface of theMCM. A layer 815 of suitable protecting agents, know as molding orglob-top materials, covers the IC chip 115 a and the bonding wires 810,protecting them; in this way, the flexible electrical interconnectionmember span 190 a becomes integral part of the electronic module. It isobserved that the wire-bond solution can as well be adopted in case acontinuous, single-span electrical interconnection member 190 isexploited.

Thanks to the present invention, herein disclosed making reference tosome, merely exemplary and non limitative embodiments thereof, it ispossible to create a propagation path, particularly adapted to sustainthe propagation of a high-speed electrical signal, from a signal sourcelocation to a signal destination location within an electronic system,avoiding most, not to say all the discontinuities usually encountered insignal propagation paths obtained through conventional techniques.

It is observed that although in the preceding description reference hasbeen made to the propagation of an electrical signal from, e.g., asignal generator integrated in a chip mounted to an MCM, to anelectro-optical converter, this is not to be intended as a limitation ofthe present invention, which applies in general to the propagation of anelectrical signal between two points whatsoever in an electronic system.In particular, and just by way of example, the invention applies as wellto the propagation of an electric signal generated by an opto-electricalconverter, like a photodetector mounted to the PCB, to a signalprocessor integrated in a chip of an MCM, or from an electroniccomponent directly mounted to the PCB to an IC integrated in a chip ofan MCM, or vice versa, or also from an electronic component embedded ina chip of one MCM of attached to the PCB to an electronic componentembedded in a chip of another MCM attached to the PCB.

It is also pointed out that although in the foregoing reference hasalways been made to a multi-chip module, this is not to be construed asa limitation of the present invention, which finds more generalapplicability, for example in cases wherein a single-chip module, with asingle IC chip like the chip 115 a assembled to a substrate like the MCMsubstrate 110, which is then assembled to the PCB 100.

1. An electronic system comprising: an electronic system supportsubstrate for the attachment of components of the electronic system,said electronic system support substrate including electric signalpropagation paths for the propagation of electric signals between thesystem components; at least a first and a second electronic components,wherein at least the first electronic component is part of a module inmechanical and electrical connection with the electronic system supportsubstrate, said module comprising a module substrate to which the firstelectronic component is at least mechanically connected, and an electriccoupling between the first and the second electronic components, theelectric coupling allowing the first and the second electroniccomponents exchange electric signals, characterized in that saidelectric coupling comprises a direct electric connection between thefirst and the second electronic components, said electric connectionbeing independent of the electronic system support substrate.
 2. Theelectronic system according to claim 1, in which said electricconnection includes a flexible electrical interconnection member havinga first end electrically connected to the first electronic component,and a second end electrically connected to the second electroniccomponent.
 3. The electronic system according to claim 2, in which theflexible electrical interconnection member includes at least oneflexible conductor strip, particularly of copper, embedded in aninsulating material.
 4. The electronic system according to claim 3, inwhich said insulating material include a material selected from thegroup including benzo-cyclo-butene, liquid-crystal-polymer, polyesters,particularly polyethyleneterephtalate, polyimides, polyperfluorocarbons,particularly polytetrafluoroethylene, aramids.
 5. The electronic systemaccording to claim 3, in which said first electronic component includesan integrated circuit chip having a contact pad, said first end of theflexible electrical interconnection member being connected to saidcontact pad by soldering.
 6. The electronic system according to claim 3,in which said first electronic component includes an integrated circuitchip having a contact pad, said contact pad being connected to aconductor strip of the module substrate, wherein said first end of theflexible electrical interconnection member electrically connected to thefirst electronic component is soldered to said conductor strip.
 7. Theelectronic system according to claim 3, in which said first electroniccomponent includes an integrated circuit chip having a contact pad, saidfirst end of the flexible electrical interconnection member beingconnected to said contact pad by means of at least one bonding wire. 8.The electronic system according claim 1 of the preceding claims, inwhich said electric connection includes at least a first span, having afirst end coinciding with a first end of the electric connection and asecond end, and a second span, having a first end coinciding with thesecond end of said electric connection and a second end, said first andsecond spans being mechanically and electrically joined one to the otherat their second ends.
 9. The electronic system according to claim 8, inwhich said first and second spans are mechanically and electricallyjoined one to the other at their second ends by soldering.
 10. Theelectronic system according to claim 9, in which said first and secondspans are mechanically and electrically joined one to the other at theirsecond ends by ball soldering, particularly micro spheres soldering. 11.The electronic system according to claim 9, in which said first andsecond spans are mechanically and electrically joined one to the otherat their second ends by means of flat pad soldering.
 12. The electronicsystem according to claim 9, in which said first and second spans aremechanically and electrically joined one to the other at their secondends by means of electrically conductive glue.
 13. The electronic systemaccording to claim 1 of the preceding claims, in which one among thefirst and second electronic components includes an electro-optical oropto-electrical conversion element.
 14. The electronic system accordingto claim 1 of the preceding claims, in which said module includes amulti-component module, particularly a multi-chip module.
 15. A methodfor assembling an electronic system comprising: providing an electronicsystem support substrate for the attachment of components of theelectronic system and including electrical signal propagation paths forthe propagation of electrical signals between the system components atleast; providing a first and a second electronic component, wherein atleast the first electronic component is attached to a module comprisinga module substrate to which the first electronic component is at leastmechanically connected, assembling the first and second electroniccomponents to the electronic system support substrate, said assemblingincluding establishing an electric coupling between the first and thesecond electronic component for the exchange of electric signals,characterized in that said establishing an electric coupling comprisesproviding a direct electric connection between the first and the secondelectronic components, said electric connection being independent of theelectronic system support substrate.
 16. The method according to claim15, in which said providing said electric connection includes providinga flexible electrical interconnection member having a first end and asecond end, and electrically connecting the first end to the firstelectronic component, and the second end to the second electroniccomponent.
 17. The method according to claim 16, in which said firstelectronic component includes an integrated circuit chip having acontact pad, said electrically connecting the first end to the firstelectronic component including soldering the first end of the flexibleconnector member to said contact pad.
 18. The method according to claim16, in which said first electronic component includes an integratedcircuit chip having a contact pad, said contact pad being connected to aconductor strip of a substrate of the module, wherein said electricallyconnecting the first end to the first electronic component includessoldering the first end of the flexible connector member to saidconductor strip.
 19. The method according to claim 16, in which saidfirst electronic component includes an integrated circuit chip having acontact pad, wherein said electrically connecting the first end to thefirst electronic component includes exploiting bonding wire.
 20. Themethod according to claim 18, comprising connecting the second end ofthe flexible connector member to the second electronic component beforemounting the multi-component module to the electronic system supportsubstrate.
 21. The method according to claim 15, in which said electricconnection includes at least a first span, having a first end and asecond end, and a second span, having a third end and a fourth end, andwherein the method includes: connecting the first end of the first spanto the first electronic component; mounting the module to the electronicsystem substrate; connecting the third end of the second span to thesecond component; assembling the second electronic component to theelectronic system substrate after said mounting the module; and joiningthe second end of the first span and the fourth end of the second spanone to another.
 22. The method according to claim 21, wherein saidjoining includes soldering or gluing by means of electrically conductivesolder or glue.